Serine Proteases
Serine proteases serve an important role in human physiology by mediating the activation of vital functions. In addition to their normal physiological function, serine proteases have been implicated in a number of pathological conditions in humans. Serine proteases are characterized by a catalytic triad consisting of aspartic acid, histidine and serine at the active site.
Naturally occurring serine protease inhibitors have been classified into families primarily on the basis of the disulfide bonding pattern and the sequence homology of the reactive site. Serine protease inhibitors, including the group known as serpins, have been found in microbes, in the tissues and fluids of plants, animals, insects and other organisms. At least nine separate, well-characterized proteins are now identified, which share the ability to inhibit the activity of various proteases. Several of the inhibitors have been grouped together, namely α1-antitrypsin-proteinase inhibitor, secretory leukocyte protease inhibitor or SLPI, antithrombin III, antichymotrypsin, C1-inhibitor, and α2-antiplasmin, which are directed against various serine proteases, i.e., leukocyte elastase, thrombin, cathepsin G, chymotrypsin, plasminogen activators, and plasmin. These inhibitors are members of the α1-antitrypsin-proteinase inhibitor class. The protein α2-macroglobulin inhibits members of all four classes of endogenous proteases: serine, cysteine, aspartic, and metalloproteases. However, other types of protease inhibitors are class specific. For example, the α1-antitrypsin-proteinase inhibitor (also known as α1-antitrypsin or AAT) and inter-alpha-trypsin inhibitor inhibit only serine proteases, α1-cysteine protease inhibitor inhibits cysteine proteases, and α1-anticollagenase inhibits collagenolytic enzymes of the metalloenzyme class.
The normal plasma concentration of AAT ranges from 1.3 to 3.5 mg/ml although it can behave as an acute phase reactant and increase 3-4-fold during host response to inflammation and/or tissue injury such as with pregnancy, acute infection, and tumors. It easily diffuses into tissue spaces and forms a 1:1 complex with target proteases, principally neutrophil elastase. Other enzymes such as trypsin, chymotrypsin, cathepsin G, plasmin, thrombin, tissue kallikrein, and factor Xa can also serve as substrates. The enzyme/inhibitor complex is then removed from circulation by binding to serpin-enzyme complex (SEC) receptor and catabolized by the liver and spleen. AAT appears to represent an important part of the defense mechanism against activity by serine proteases.
α1-antitrypsin is one of few naturally occurring mammalian serine protease inhibitors currently approved for the clinical therapy of protease imbalance. Therapeutic α1-antitrypsin has been commercially available since the mid 1980's and is prepared by various purification methods (see for example Bollen et al., U.S. Pat. No. 4,629,567; Thompson et al., U.S. Pat. Nos. 4,760,130; 5,616,693; WO 98/56821). Prolastin is a trademark for a purified variant of α1-antitrypsin and is currently sold by Talecris Company (U.S. Pat. No. 5,610,285 Lebing et al., Mar. 11, 1997). Recombinant unmodified and mutant variants of α1-antitrypsin produced by genetic engineering methods are also known (U.S. Pat. No. 4,711,848); methods of use are also known, e.g., (α1-antitrypsin gene therapy/delivery) (U.S. Pat. No. 5,399,346).
Graft Rejection
There are many diseases that culminate in organ dysfunction or failure. Representative non-limiting examples include renal failure due to diabetes mellitus, hypertension, urinary output obstruction, drug-induced toxicity, or hypoperfusion, as well as cardiac dysfunction due to ischemic coronary artery disease, cardiomyopathy/infection, or valvulopathy. Pulmonary diseases include substantial damage due to chronic obstructive pulmonary disease (COPD, including chronic bronchitis and emphysema), AAT deficiency, cystic fibrosis, and interstitial fibrosis. Under certain conditions, the only therapeutic option for treatment of a subject may be organ transplantation. Pancreatic-islet transplantation provides diabetic patients with the only option for a tightly-controlled blood glucose level, as proven to be essential for prevention of diabetic complications. In the case of islets, post-transplant inflammation, which precedes immune rejection, is a critical determinant of graft survival. This early inflammation is mediated by cells other than the impending allospecific immune cells.
One challenge to therapeutic transplantation is the damaging effects of the host immune system on the transplant. MHC molecules exist on the surfaces of cells and the particular structures of MHC molecules are typically unique for each individual (with the exception of identical twins, where the MHC molecule complements are identical). The immune system is programmed to attack foreign or “non-self’ MHC-bearing tissues. For these reasons, when an organ or tissue is transplanted into a recipient, an effort is made to optimize the degree of tissue matching between donor and recipient. MHC antigens are characterized for the recipient and donors. Matching a donor to an allograft recipient by MHC structure reduces the magnitude of the rejection response. An archetypal example is blood group matching. Most transplants are allografts that occur between non-identical members of the same species. Since these matches are imperfect, there is an expected graft rejection immune response associated with allografts. Current methods used, in order to enhance graft survival, include medications to suppress the immune response which can result in graft rejection. These medications are referred to immunosuppressant or antirejection drugs, such as prednisone, cyclosporine A, and cyclophosphamide, to name a few. As mentioned above, local inflammation is experienced immediately after grafting, and cells that are particularly sensitive to non-specific inflammation, such as islets, can endure graft dysfunction more severely than other types.
Despite advances in the field of antirejection therapy, graft maintenance remains a challenge since the available antirejection therapies are imperfect. For example, immunosuppression enhances the risk for opportunistic infection or neoplasia. Toxicities abound and include, but are not limited to, diabetes, organ dysfunction, renal failure, hepatic dysfunction, hematological defects, neuromuscular and psychiatric side effects, and many others. Therefore, there is a need for a more effective anti-rejection medical treatment that prolong graft survival and improve the quality of life.
Bone marrow transplantation is a unique kind of transplant where immune cells from a donor are transferred into a recipient, thereby conferring the donor immune system into the recipient. Here, the graft is capable of generating an immune response against the host, and this is termed “graft versus host” disease (GVHD). Immunosuppressive and antimicrobial treatment is required to block adverse consequences of GVHD, and a need exists for safer and more effective inhibitors of the adverse effects by the graft.
Because of some of the difficulties and inadequacies of conventional therapy for treating transplantation complications and associated side-effects, new therapeutic modalities are needed.